1. Technical Field
The present invention relates to electric motor control.
2. Related Art
Electric motors have load torque-rotational speed characteristics (T-N characteristics). The T-N characteristics are inherent to the electric motors, and basically the T-N characteristics of a certain electric motor is uniquely determined if the drive voltage is determined. However, the T-N characteristics can be changed using a control technology called field-weakening control (e.g., JP-A-2002-320400).
The conventional field-weakening control is realized by controlling the d-axis current to a negative value taking the speed and the load into consideration. However, since the voltage with which the electric motor is used depends on the use circumstances of the user, it is not achievable to sufficiently improve the T-N characteristics of the electric motor.